Not to be rude, but there was some pretty backwards stuff posted in here.
1: The explorer 8.8 is definitely not a bolt in deal. The Ranger 8.8 will bolt up, but he will have to drill and tap the yoke in order for the drive shaft to match up, otherwise, he would have to pick up a 1990 and up drive shaft from a 4.0. The 83-89 had #1210 u-joints. After that they uses 1310 or 1330, depending on how the original owner ordered it. The 1210 will not fit the 8.8 yoke, however some 7.5's had the #1310, which had the larger yoke, that will bolt up.
2: If he wants lower RPM on the hwy, 4.10's are certainly not the way to go. Since he has 3.73's he would want to go with 3.55's.
3: The 2.9 is a very small V-6, and small v6's like higher RPM's. If you're doing 65 @3500 RPM and you hit a long hill, you're probably okay. If you're doing 65MPH, and hit a long hill at 2600RPM, you'll be downshifting before you know it.
4: Being that it's a smaller V-6, and likes/needs the higher RPM to move it's self, you will lose gas millage due to bogging the engine, and more down shifting. When engines bog, they're not running at their maximum efficiency, and is VERY hard on them.
Just some of my thoughts. Take them for what you will. Always remember: Lower RPM doesn't always mean more MPG!
Regards~
-Denny